This invention relates to cellulose microspheres and a method of manufacturing them.
Particles of cellulose and its derivatives (hereinafter referred to as "cellulose particles") are used as ion exchangers, packing materials for chromatography, adsorbents for metal or protein, additives for cosmetics, carriers for immobilization of biocatalysts and for a variety of other purposes.
Cellulose particles are manufactured in many different ways, which are roughly divided into physical and chemical methods.
In one physical method, viscose is extruded from a discharge nozzle so that its flow will change from a continuous flow to a droplet flow while dropping, and dropped into a coagulating/regenerating bath in the form of substantially spherical droplets to manufacture regenerated cellulose particles having a particle size of 16-170 mesh (88-1168 .mu.m) (examined Japanese patent publication 56-21761).
In another prior method, cellulose particles are manufactured by spraying viscose into hot air to dry it by use of an atomizer such as a twin-fluid atomizer or a rotary nozzle (unexamined Japanese patent publication 4-41533).
Chemical methods include one comprising the steps of suspending viscose in a water-immiscible solvent such as chlorobenzene, solidifying it by heating to a temperature of 30-100.degree. C. while continuously stirring the suspension, and acid-treating the thus formed particles to manufacture cellulose particles having a particle size of 80-1000 .mu.m (examined Japanese patent publication 57-45254).
Also disclosed is a method comprising the steps of dispersing viscose in a water-immiscible solvent such as cyclohexane, preparing a stable W/O type viscose emulsion using a surfactant, and reacting the emulsion with a W/O type emulsion of coagulating solution prepared by adding a coagulating solution and a surfactant to a water-immiscible solvent by mixing them together, or blowing an acid gas into the W/O type viscose emulsion or directly adding a coagulating solution into the W/O type viscose emulsion to manufacture cellulose particles having a mean particle size of 15 .mu.m or less (unexamined Japanese patent publication 5-200268).
There is another method comprising the steps of cutting filaments of cellulose triacetate manufactured by dry spinning from a methylene chloride or chloroform solution of cellulose triacetate into chips, heating the chips in such a medium as silicone oil to melt them, and saponifying them to manufacture cellulose particles having a particle size of 30 to 500 .mu.m (examined Japanese patent publication 55-39565).
Another prior art method comprises the steps of preparing a dispersion of minute viscose droplets by dispersing viscose in an aqueous solution containing a water-soluble anionic polymer, coagulating the particles by heating the dispersion or mixing a viscose coagulant, and regenerating the cellulose particles with an acid to manufacture cellulose particles having a mean particle size of 20 .mu.m or less (examined Japanese patent publication 5-76496).
The abovesaid physical methods can be carried out using devices having a simple structure and permit continuous production, but have a problem in that it is difficult to obtain spherical particles because a large amount of particles of indeterminate form are produced in a sprayed state in which droplets of a cellulose solution are dispersed in an air flow, that air bubbles tend to mix into the cellulose particles and that the particle size distribution tends to be large.
In chemical methods in which a water-immiscible solvent is used, the steps of removing the water-immiscible solvent and washing the cellulose particles are needed to separate the particles formed. Further, since an organic solvent is used as the water-immiscible solvent, these methods are undesirable from safety, environmental and economical viewpoints.
On the other hand, the method in which chips of cellulose triacetate are melted needs many steps such as melting and spinning cellulose triacetate, cutting it into chips, and melting the chips. Also, the manufacturing efficiency is thus low.
Further, in a method in which a cellulose-rich phase is separated as droplets from a liquid mixture of viscose and a water-soluble anionic polymer by the interaction of both components, no complete phase separation occurs, i.e. the mixture never separates into one phase containing cellulose only and another containing water-soluble anionic polymer only. Rather, the mixture is simply separated into a cellulose-rich phase and an anionic polymer-rich phase under a specific partition coefficient. Thus, there remain water-soluble anionic polymers in the cellulose particles obtained. From the fact that regenerated cellulose membranes are used as dialysis membranes, it is apparent that it is difficult to remove any water-soluble anionic synthetic polymers, which are high in average molecular weight, from the cellulose particles by washing.
An object of this invention is to provide a method of manufacturing cellulose microspheres which is free of these problems and high in productivity, and which takes into consideration the safety and environmental aspects.